Assuming no losses and inefficiency, the absolute maximum USB2 throughput is 60Mb/s.
The maximum 16x DVD transfer rate is 22Mb/s
So theoretically, USB2 could simultaneously run:
2 drives at 16x
3 drives at 12x
5 drives at 8x
USB2-IDE interfaces tend to be supplied with a PSU for the drive, but not sure if this also powers the interface (zero USB power), or if the interface is a standard low power (100mA) or high power (500mA) device.
For anything other than high power devices (at one hub depth) an unpowered hub is ok - if you chain hubs more than one deep (hub into hub), then loading more than 5 ports at low power will exceed the limit.